To accurately control the flow of fluids such as fluid control valves in pneumatic conveying system, it is usually necessary to have a fluid metering valve that can be adjusted to precisely meter the correct amount of fluid flowing into the system. One of the difficulties with prior art control valves is that the flow rate through the valve is generally not proportional to amount of rotation on the flow control knob. That is, if the valve is nearly closed, one revolution of the flow control knob may increase the flow rate by one unit, but if the valve is nearly half open one revolution of the control knob may increase the flow rate through the valve by two units. Thus the change in flow rate through the valve is dependent on the flow condition of the valve; namely, whether the valve is in a nearly closed condition or a fully open condition. The present invention provides a fluid-control valve which provides a linear relationship between the fluid flow rate and the rotation of the flow-control knob. Consequently, regardless of the flow condition of the valve, one rotation of the flow control knob on the valve provides a corresponding one unit increase or decrease in the flow rate through the valve. In addition, the unit is readily disassembled for cleaning, can be made from polymer plastics and includes safety features that prevent the valve from injuring a party if the valve is accidentally disassembled while under pressure.